Electric heating system



y 7, 1963 P. EISLER 3,089,017

ELECTRIC HEATING SYSTEM Filed July 31, 1958 4 Sheets-Sheet l wws/vropPau/Eis/er' BY 4 {M A T TOPNEY y 7, 1963 P. EISLER 3,089,017

ELECTRIC HEATING SYSTEM Filed July 31, 1958 4 Sheets-Sheet 2 Fig.3.

E94 1 H 5, r Qqi I E 2 H c an E E a E 1 I I l I l i i I 1 I /N VE N TOPPaul [is/er BY .4 Zfl/z A TTORNEY May 7, 1963 P, ElsL 3,089,017

ELECTRIC YSTEM May 7, 1963 P. EISLER ELECTRIC HEATING SYSTEM '4Sheets-Sheet 4 Filed July 51, 1958 /N V E N TOR Paul [ls/er UnitedStates Patent Ofifice Fatented May 7, 1963 3,689,017 ELECTRIC HEATHNGSYSTEM Paul Eisler, 57 Exeter Road, London NW. 2, England Filed July 31,1958, Ser. No. 752,384 Claims priority, application Great Britain Aug.6, N57 '7 Claims. (Ql. ZEN-20) The present invention relates to electricstep down transformers for use in electrical heating whether for spaceheating or for heating objects or materials such as water or foodstuffs.

In the ordinary way the function of a transformer is to change thevoltage of an alternating current with as little loss as possiblecompatible with reasonable cost and sound construction. For this reason,transformers normally have a cubic form with high space factor for thecore and winding, and are usually made of relatively costly materialsuch as silicon steel for the core which has low hysteresis loss.

The losses in a transformer go up with reduction in the quantity andquality of material in the core; they also go up if the cross section ofthe conductors is reduced. The losses moreover appear as heat. If nowthe whole purpose of the transformer is to form part of a heatingsystem, provided the heat can be applied where wanted and thetemperatures involved are suited to the purpose it is no disadvantage ifpart of the useful heat is developed as loss in the transformer, and incertain directions at any rate this can reduce the cost of thetransformer not so much by reducing the quantity of material used as bymaking it possible to use material of cheaper quality as is in fact donein the present invention.

The purpose of using a transformer at all is to enable the heating(apart from the heat produced in the transformer itself) to be effectedin a low voltage circuit the voltage in which is below a value which isconsidered dangerous to touch. Accordingly the secondary voltage is madeless than, and usually considerably less than say 48 volts betweenconductors and between conductors and earth. The secondary winding willbe completely insulated and separated from the mains. In view of thepurpose of the transformer the primary windings is made suitable forconnection to conventional mains in which the supply may be at 100600volts more usually 110 or 220/250 or 440 volts at 50 cycles.

Briefly stated therefore, the present invention provides an electricalheating system comprising an electrical stepdown transformer, saidtransformer comprising a primary winding for connection across the mainsand a low voltage secondary winding, the transformer dissipating about20-33% of the electrical energy supplied thereto in the form of heat,means in heat-conductive contact with the transformer for radiating theheat so dissipated, and a low voltage electrical heating resistanceconnected across the said secondary winding.

As well as serving @as a space heating radiator a transformer accordingto the present invention is very suitable for use as a hot plate orwarmplate in connection with the heating up of food and keeping food hotor warm once it has been heated.

Because a transformer used according to the present invention developsconsiderable heat within itself but has a high surface to volume ratio,it can be used to operate a heating cycle which gives very rapidproduction of heat at first with a more or less steep or gradual fallingofl. Such a cycle will conveniently directly heat food stuffs and thenmaintain them hot. Furthermore it can produce a secondary voltage havinga similar character, which is useful for the same purpose.

The invention will be further described with reference 2 to theaccompanying diagrammatic drawings which illustrate some embodiments ofthe transformer and possible modifications thereof as well as one formof circuit in which it may be used. It will be understood that thedrawings are given by way of example and that the invention is notconfined by them.

FIGURES 1 and 2 are circuit diagrams showing how a heating systemaccording to the invention may be set up.

FIGURE 3 is a part sectional View of a stage in the construction of afirst embodiment of transformer.

FIGURE 4 shows in side view the completion of the transformer shown inFIGURE 3 and its embodiment in a space heating radiator.

FIGURE 5 is a similar view to FIGURE 4 of an alternative arrangement ofthe same basic form of transformer.

FIGURE 6 shows another embodiment of space heating radiatorincorporating the same form of transformer as FIGURE 4.

FIGURE 7 is a plan view of a second embodiment of transformer accordingto the invention.

FIGURE 8 is a perspective view illustrating a stage in the assembly ofthe transformer of FIGURE 6.

FIGURE 9 is a side view of the third embodiment of transformer accordingto the invention.

FIGURE 10 is a plan view of FIGURE 9, and

FIGURE 11 is a side view of another embodiment of transformer accordingto the invention.

FIGURE 1 shows a typical heating system according to the invention. Theprimary coil is designed to be energised from a normal mains supply sayat 240 volts 50 cycles. The secondary coil comprises two windings "11,12 which can be put in series and parallel by a switch 13. The primarycoil may have tappings for regulation purposes operated on the mainsside in a conventional manner while the secondary side is controlled bya resistor 15 or tape form which can be extended or which rolls up forinstance under spring tension like a measuring tape. Only the extendedlength is in the circuit either in series with the main external load 14or on the full secondary voltage constituting itself a minor ex ternalload that is a heating tape used for instance as an alternative smallheater instead of the large external heater 14.

The switch-over is effected by switch 16 which has three positions: Inthe centre position {shown in FIG- URE 1) resistor 15 is in series withload 14 permitting a continuous variation of the ohmic resistance of thetotal load on the transformer. For full working of load 14 the taperesistor is completely rolled up. The lamp 19 between the two contacts15b is then dark while the lamp over load 14 is bright. If the switch 16is in top position, load 14 is cut off and the tape resistor 15 isacross the secondary transformer terminals which are connected either inseries or parallel according to the position of switch 13. The topposition of switch 16 is only possible when the tape 14 is extended atleast a certain length so that it cannot run too hot. This is ensured bya mechanical interlocking arrangement symbolized in FIGURE 1 by the bar15a moving with the free tape end and bearing against a quadrant 16a onswitch 16, thus preventing an upward movement of switch 16 until it haspassed, that is until the minimum tape length is extended. The lower barend cannot pass the plunger 16a when the tape resistor 15 rolls up andthe bar 15a remains in front of the switch but allows the tape 15 toroll up completely. In the bottom position of switch 16 all loads areoff.

The tape resistor 15 is only indicated schematically as a resistor goand return line; in practice it is a heating film of the type describedin my co-pending applications Serial No. 747,315, filed I uly 8, 8, nowPatent 3 No. 3,033,970, and Serial No. 783,633, filed November 10, 1958,now Patent No. 3,020,378. The contacts 1512 are preferably rollers.

The function of bar 15a is not only to prevent switch 16 being movedinto an undesirable position, but also to prevent any switching beingeifected at all on the secondary side, on either switch 13 or 16, unlessthe tape 15 is fully or nearly fully extended. This further lockingarrangement is indicated by sliding door 17. As long as the bar 15a isnot holding it open against the force of a spring 17a, it covers up bothswitches 13 and 16 which therefore cannot be operated.

Naturally these interlocking means are only symbolically indicated inFIGURE 1 and any of the known practical locking devices can be use-d forthe symbolized arrangement. The purpose is to ensure that the current atthe switch contacts when the switches are operated is reduced to nil orto such a small value that very cheap and light duty switch contacts cansafely and reliably be used.

There are fuses 18, indicator lamps 19, both primary and secondary, athermostat 20 or a temperature controlled automatically resettingcircuit breaker and proper earthing connections. When at the beginningof a heating cycle switch 13 is in the series position the transformeris heavily overloaded for a very short time. In this case the secondarysupply voltage is twice the usual voltage which is obtained when bothcoils 11 and 12 are in parallel and consequently the wattage isquadrupled. The transformer and the external heating device 14 willconsequently heat up very quickly and the device 20 will switch themains supply off temporarily when the desired temperature is reached. Assoon as this happens, the current supply to coil 8 via leads 9 isswitched off. The flux change within the coil exerts a forward force onthe bar 10 connected to blocking plunger 13c and situated within thecoil. The blocking plunger 13c drops and permits the spring 13a to pullthe switch 13 into the parallel position shown in FIGURE 1 where itremains when the device 20 closes the primary circuit again. Spring 13aand plunger 13c are shown here only by way of symbols for standardblocking devices with proper resetting provisions. The latter can bemanual or automatic when the temperature has dropped to a pre-set level.

FIGURE 2 is a simplified circuit diagram of a heating system with atransformer having two primary coils 11a, 12a. The system operates withthe switch 13- at first parallelling both coils; as soon as thethermostat or temperature controlled automatically resetting circuitbreaker 20 breaks the primary circuit for the first time the switch 13is pulled over into the position in which it connects both coils inseries, thus reducing the load to A of what it was before. The resettingof switch 13 for parallel connection of the coils is done manually orautomatically when the transformer has cooled down sufficiently. Thecircuit gives a heating cycle suitable for very rapid heating up wherethe mains supply has sufficient capacity. This cycle can be made stillmore steep if two primary and two secondary coils are provided in seriesparallel connection giving load ratios of 16:4:1.

By Way of example for space heating the transformer may be a singlephase mains transformer of 3 kva. made as a fiat panel which dissipatesabout 1 kw. as heat for space heating while the secondary output of 2kw. is supplied usually at about 12 volts to a wall-, ceiling, orfloor-covering, hereafter called heated wallpaper, which consists of analuminium foil pattern on a thin layer of plastic film or paper orbetween two such layers. This and other heated wallpapers and otherheating devices which can be used in the heating system of the inventionare described in my copending applications Serial No. 747,314, filedJuly 8, 1958, now Patent No. 2,971,073, Serial No. 747,315, filed July8, 1958, now Patent No. 3,033,970, Serial No. 783,633, filed November10, 1958,

4- now Patent No. 3,020,378, and Serial No. 783,609 filed December 27,1958.

The core and coil cross sections are initially and similarly calculatedas for a 0 core transformer with the difference that (a) the coreconsists usually of very thin, flexible and grain oriented, tapes ofvery cheap grade steel foil giving high loss instead of the usual grainoriented, high grade magnetic strips and the windings consist ofaluminium foil instead of copper wires, that (b) the combined coil andcore losses at full load can reach say 20 to 33 percent.

Compared with conventional transformers and taking the diiferentmaterials into consideration the smaller active cross sectional areas ofthe windings and core tend to reduce the weight, while the arrangementsnecessary to give them thin flat shapes, to be described below, tend tohave the opposite effect.

The core, see FIGURES 3 and 4, consists of several layers of the thin,highly flexible steel foil tapes 21. The number of layers which can besuperimposed on each other, that is the thickness of the stack, is notonly restricted by the desired loss and heat dissipation requirements ofthe present invention, but also by its requirement that the core stackprior to its assembly with the coils and its closure must be veryflexible, like a belt. It is for convenience called a belt hereafter. Tokeep the drawing clear the thickness of the layers has been exaggeratedand their number reduced.

While the cross section of a conventional 0 core is a square or aslightly elongated rectangle, the core cross section of the presenttransformer is so fiat that its long side (:width of core) which is thewidth of the steel foil or the combined width of narrower paralleledsteel foil tapes plus installation between the tape edges is a highmultiple of thickness of the foil stack or core. All or some of thesteel foils may be slit along their length if, in a particular case orfor some only of the foils of a core, the eddy current loss is thoughtto be too great.

The coils are wound over a removable former of the core cross section sothat they Will be able to slide like sleeves over the core belt. Theyare wound from tapes of aluminium foil with a thin impregnated paper orplastic film tapes 22 interleaved between turns. This paper or filmextends over the foil edges. The primary Windings consist of a tape withseveral parallel strands 23 of aluminilun foil and this tape can beproduced as a tape cable element by any of several well known mechanicalor printed circuit methods. The inner and outer tape ends are taken outsideways, interconnected, in series .or series parallel to provide thenecessary electrical paths, provided with terminals and sealed over withinsulation. The secondary winding which requires fewer turns but greatercross section consists of tape with a single wide strand 24 of aluminiumfoil the inner and outer sides again being taken out sideways. By thismeans the primary and secondary windings can be made of substantiallyequal depth i.e. dimension nonnal to the foil of the core where itpasses through the winding. The two coils may also be made ofsubstantially equal width. In the drawings the thickness of the foilsand insulation have been exaggerated and the number of layers reducedfor the sake of clearness.

Impregnated paper or plastic film tape 25-wider than the foilsmay alsobe used to insulate the superimposed layers 21 of the core and to bondthese layers into the long belt-like unit except at both ends of thebelt. The interleaving insulating tapes 25 are not cured at the frontend and there only hold the foils temporarily together by adhesion. Thusthe front end can be opened and the first few inches of each foil layer21 then become accessible. The insulating tapes 25 stop short of therear end of the belt so that there the foils are free and bare on bothsides.

The assembly of the two coils with the core belt is effected by simplyslipping the coils over the belt and joining the belt ends layer bylayer, the bare rear end of each layer 21 contacting the non-insulatedface of its front end. The joint-with all ends superimposedis pressedtogether and cured by heat generated for instance by supplying the coilswith current.

The bonding of the belt ends by means of the interleaving insulatingtapes 25 may be supplemented or replaced by addition of other cements.Generally the bond is additionally secured by a mechanical fixture,clamp, bolts or the like. For this purpose the foils 2 1 at the beltends may be perforated with oblong holes or other cutouts.

It is to be noted that the closed belt with the coils loosely threadedon is spacious and flexible and very convenient and easy to Work onmanually and with tools or machines.

The description has so far referred to a single belt for onetransformer. Under mass-production conditions the belt can be producedin continuous lengths limited only by the length in which the foil canbe made and laminated fully automatically by a fairly simple machinesimilar to a paper converting machine. In a similar way many coils canbe wound on one axis simultaneously or sliced from one very longcylindrical coil as described in British patent specification No.700,451; and the coils can be slipped over the continuous belt before itis cut into lengths for single transformers.

The joining and securing of both belt ends can also be done by theautomatic machinery. Thus labour cost in transformer production can bereduced considerably and high accuracy can be obtained.

The next step in the transformer production is to stretch the closedbelt with the coils on it into a flat panel shape and enclose it betweentwo anodized aluminium or steel sheets. The sheets are suitablydecorated for space heating radiator purposes and are associated with astand or legs and frames. In the preferred method as shown in FIGURE 4one coil is placed close to the joint indicated at 26 and the other atthe diametrically opposite region when the belt is imagined as circle,and then both coils are moved relatively until the belt-free flat coilsurface of one coil is in line With the outer surface of the belt whichis pressed against the flat surface of the other coil, as shown inFIGURE 3. The belt is so dimensioned that in this position it fits verytightly over the coils.

The joint 26 lies in and preferably fills the space between the coilsand thus spaces them apart. This arrangement of the coils on oppositehalves of the belt also separates the two parts of the belt which runinside the coils and gives more uniform temperature conditions. Thermalexpansion of the belt for example when assembling the transformer withthe enclosing sheets while the belt is heated may be used to ensure atight fit of the belt. But if it is slack it may be tensioned byinsertion of wedges and distancing strips.

When a clamping device is used for securing the joint 26 of the beltwhich features an eccentric bolt or pin, this can also be usedadvantageously for tightening the belt at this juncture. Tightness orslackness beyond the capacity of these or similarly convenientadjustment facilities require lengthening or shortening of the belt byresetting the joint and indicates an inaccurate dimensioning of the beltlength which should be corrected for subsequent production. However, inany particular case if the belt is too short instead of re-setting thejoint, the coils may be shifted to bring the joint beyond the outer endof one coil thus enabling the facing ends of the cores to be broughtcloser together. It would also be possible to put both coils on the samehalf of the belt and arrive at the configuration of FIGURE 5 or byshifting the joint to one end to bring the coils still closer together.This configuration can indeed be deliberately chosen but the temperatureconditions in the core are not so favourable as in the configuration ofFIGURE 4. When either configuration is sandwiched between two decoratedmetal sheets 27, 28 the total thickness is essentially equal to thethickness of the decorated sheets plus the thickness of the core beltplus the thickness of the coil which in turn and excluding play is thebelt thickness plus twice the radial thickness of the cross section ofthe coil windings i.e. the thickness normal to the foil of the corebelt. Thus the total thickness is a little more than twice the thicknessof core and coil cross section. Usually it is just over four times thecore thickness.

The decorated metal sheets 27, 2-8 hold the transformer under thepressure of bolts, screws or other fixing devices which connect bothsheets and keep the sandwich together. The sheets are earthed throughtheir large area of contact with the earthed belt and by additionalearth contacts.

In order to minimize noise, potting or other encapsulation of coils andcore can be carried out with a minimum of resin and without interferingwith the heat flow to the metal sheets through their large contact areaswith the coils and core. The resin can be filled with aluminium powderor the like to improve its heat conduction.

Instead of sandwiching the transformer between two metal sheets it canbe fixed, for example by clamping bolts, to only one decorated sheet 27,FIGURE 5, which forms the front of the radiator while the back sheet isdistant from the rear surface of the transformer by a small air gap 31.The top of the rear sheet 2-8 has a curved hinged lid 32, which operatesa switch and a resistor fixed on the transformer as a radiator. Openingthe lid switches the main secondary load (wallpaper) on by graduallyincreasing the value of the resistor across the secondary terminals,switches a decorative indicator light on and allows an airstrearn toflow through the gap between rear sheet 28 and rear surface of thetransformer to the top of the sheets and into the room. This heatconvection does not take place when the hinged lid is fully closed andthe normal secondary load is off.

With no secondary load the transformer would however run too cool and beuseful only for drying clothes or similar purposes and not as a spaceheating radiator of l kw. The lid therefore has two open positions. Thesecond open position so adjusts the resistor across the secondaryterminals that the combined transformer loss and the load presented bythe resistor amounts to 'l kw. which the panel must dissipate. Theairflow is not interfered with in the second open position, but theindicator lights are dimmed.

The description of the example of transformer con struction according tothe present invention shown in FIGURES 2 to 5 has referred only to theuse of ordinary aluminium and steel foil and to impregnated paper andplastic film as insulants. These materials are perfectly suitable aslong as the design by adequate dimensioning of cross sectional andsurface areas ensures that at no point a temperature will occur abovethe limits permissible for these insulants. For so called lowtemperature radiators running below C. this condition is readilyobtainable. For higher temperature work however, be it inside the panelor on the surface of the panel, variations of insulants and/or foils areproposed which will be described later after the description of afurther example of the basic construction of the transformer.

The second example is also a transformer constituting and used as a thinflat heating panel. Its conventional analogue is a single-phase,shell-cored, double wound transformer with the primary and secondarywindings on the same limb.

Whereas in the first examples the coils were wound with very wide andthin (aluminium) foil tapes, the coil of the second example is made fromvery narrow foil tape. The axial length of the complete coil is aboutequal to the tape width if the primary and secondary windings are sowound that they are radially superimposed one over the other. If bothwindings are wound side by side on the same size former and have thesame radial depth, the axial length of the complete coil is then the sumof the widths of the tapes of both windings. The complete coil has theshape of a very elongated oval with straight long sides, and is marked41 in FIGURES 6 to 8. It can readily be mass-produced, for instance bywinding a great number of coils on one long mandrel.

The opening between the straight long sides can be slightly widened ormade narrower in order to facilitate the method of assembly of the coiland core proposed by the invention. The elasticity of the coil in thisdirection is provided by crimping the tapes during winding near the endsof the oval or by elastic interleavings inserted there during windingbetween given numbers of turns. The elasticity of the coil is notrequired after final assembly, and does not therefore stand in the wayof an encapsulating, potting or curing operating at a later stage.

There are two varieties of core for this coil. The one illustrated inFIGURES 7 and 8 uses the three-limb laminations of heavier gauge, butstill flexible steel foil, the other illustrated in FIGURES 9 and 10consists of preformed and bonded stacks of steel foil which can be bentin the middle and have each foil accessible at the ends prior to andduring assembly.

Referring now to the construction illustrated in FIG- URES 7 and 8 thesecore laminations are made from lacquered narrow steel tape by punchingtwo slots 42, 43 and three holes 44 in each length which is parted offthe tape by severing at 45 the middle limb 46 from one of the two longlinks of each lamination. The three holes 4 4 are in the centres of thelimbs, two large holes in the centre limbs and a small hole in themiddle limb. A few end plates of stiff heavy gauge steel are also prepared, not necessarily with a middle limb, if machining them forinsertion of a separate middle member during core assembly is moreconvenient than bending their middle limbs.

The core assembly is effected by sliding the laminations over the coilfrom the end as illustrated in FIGURE 8. The middle limb 46 of thelamination is flexed up to slide over the end of the coil, the straightlong sides of which have a fair clearance in the slots 42, 43 of thestampings. The centre limb 4-6 of each lamination springs back into theplane of the lamination as soon as it reaches the coil opening. Thelamination can be assembled with the cut in the central limb at the topand bottom alternately. Stacks of the flexible lamination are packedbetween the stitf end plates which are marked 47. Neighbouring endplates 47 are spaced by washers 48 to leave cooling channels 49. InFIGURE 7 three stacks with two such channels are shown, but obviouslyany number can be produced. Two insulated long bolts pass through theholes in the outer limbs of laminations and hold the core together roundits perimeter. Direct clamping pressure on the middle limbs of thelaminations can be obtained by flexible members which are tightened andsecured while tight. The flexible member shown in FIGURE 7 is aninsulated wire or wire rope 50 with a lead on one end. It is threadedthrough the holes in the middle limbs and washers between the stacks andis tightened by its other end being wound on a pin 51 held on the lastend plate. This pin can be turned by a screw driver or spanner and besecured against unwinding by a ratchet device 51a acting on the head ofthe pin.

If the middle limb is provided with more than one hole or if there otherpathways are provided (for instance notches) for flexible tighteningmeans the described wire or wire rope may be replaced for instance byloops of wire which encircle the limbs in each stack, the loops beingtightened by twisting their ends together or drawing them tight by ascrew device. This tightening can be effected as each stack isassembled, the loopend being accessible in the opening of the coil. Evenwhen the last stack is assembled there is room in the semicircular endportion of the opening. Alternatively to flexible members, eccentricmembers lodged in the semicircular end parts of the coil opening may beused for tightening the core stacks, or if the cuts 45 are alternatelyat the top and bottom, two further long bolts of small diameter might beused at the centres of the long links of the core lamination. The coilplay in the slots may be eliminated by the wedging action of aneccentric device or long wedge inserted in the outer ends of the slots.In FIGURES 7 and 8 the thicknesses have been exaggerated and numbers oflayers reduced for the sake of clearriess.

The other variety of core illustrated in FIGURES 9 and 10 is made fromwide thin steel foil tapes having a plastic cement coating on one sidebut with narrow strips across the foil width left bare at regularintervals. The foils are cut at the centres of these uncoated strips.The coated length of different foil tapes, that is the spacing of thestrips, varies according to the place the cut length of foil is to takein a particular core. The further from one side of the core opening, thewider the spacing. The width of the cut steel tapes is almost equal tothe length of the parallel sided part of the coil opening.

A thin stack of the cut foils 61 is assembled in a kind of mould whichforms and part cures it into a long narrow U shape. The space betweenthe limbs of the U closely fits the said length of the coil. The bend ofthe U is not fully cured and remains flexible and the ends of the limbswhich were free of cement do not adhere to one another. The inner layersof the limbs are shorter than the outer ones, and the cement edgefollows the same length.

The assembly of the coil with two such U parts is done by bending onearm of the U about degrees as shown in chain lines on the right handside of FIGURE 9, inserting this arm through the elastically widenedcoil opening, bending the arm back again into the U shape and pressingthe coil together again. This process is done with both U parts formingthe core either simultaneously or in succession. A bonding layer, cementand/or insulating tape indicated at 62 has been placed on both faces ofthe coil prior to assembly with the U, so that a press-curing in thefinal assembly stage bonds core and coil together on all sides. Beforethis final stage is reached, there are however some more operations.

The next step is to join the free ends of the U limbs over the straightsides of the outside turns of the coil. Starting with the inner foilends of the U each pair of ends is bent over, superposed and if desiredjoined together, and a cement and extra insulating layer indicated bythe numeral 63 is applied between each pair of foil ends. In FIGURE 9the thicknesses of the layers have been exaggerated and the numbers oflayers reduced for the sake of clearness. A steel tape 64 going roundthe outside of the core and coil holds the joint under pressure whilewedges of the eccentric type which were placed along the inside and/oroutside of the long straight sides of the coil tighten up the wholeassembly. The wedges along the inside turns may actually be tubes filledwith an expanding medium acting like a thermal fuse liquid or athermostat which switches for instance a part of the load on and off.They may be airfilled pneumatic tubes 65 made of a heat-resistingelastomer inside the coil and/or on the outside coil windings solely fortightening the core after assembly. Terminals or fixing screw arepreferably arranged at both axial ends of the core.

Very thin transformer structures can be made in accordance with FIGURES7 to 10. The transformer can be mounted as described with reference toFIGURES 4 and 6'. Its decorative treatment depends on the temperature atwhich is desired to run this heating radiator. If the temperature stillpermits plastic insulation, the transformer can be potted with a formedsheet as mould to save resin, and a decorative print can be placed justunder the resin surface of the front face. Or the decoration can be doneby laminating as in various known decorative laminated products having aplain or patterned coloured, fabric-like or wood grained surface. Forhigher temperature running the transformer can be placed in a fiat oiltank the surface of which is then the immediate heating panel, or it canbe used without cladding on a stand in front of a reflector. Such oilimmersion is also possible with the transformer constructed as inFIGURES 3 to 6.

FIGURE 11 shows a transformer according to the invention adapted toserve particularly for heating food. The transformer itself has a core66 and a winding '67 arranged similarly to a conventional shell typetransformer except that as before the whole structure is made of veryflat proportions and the core is built up of very thin flexiblematerial; also cheap materials are used for the core and windings are ofaluminium foil. Although the core is built up of pieces to permitassembly there are no bolt holes but the outer limbs are folded over asshown at 68 so that the total height at the side approximates that ofthe windings 67. Thus it can be firmly clamped by means of a stout plate69 and bolts 71 which clear the core and press the core against theinturned flanges of a potlike member 72. The flange may if desired bereplaced by a complete bottom closure of the member 72 but this is notnecessary. If the core at 6 8 is not of sufiicient depth packing can beincluded. At the top the member 72 is provided with a hinged lid 73 anda hook 74 is formed on its front edge so that the package of food can besuspended when the lid is raised. For the convenience of the user ofthis structure, it is mounted on a tray 74.

I claim:

1. A transformer assembly comprising a plate-shaped electrical step-downtransformer having a primary and a secondary winding, means in heatconductive contact with the transformer for radiating heat dissipatedwithin the transformer and control means for causing the transformer tooperate under a predetermined initial load for a predetermined periodand for subsequently decreasing the load and maintaining it at asubstantially constant, reduced value.

2. A transformer assembly comprising a plate-shaped electrical step-downtransformer, said transformer comprising a belt-shaped flexible core ofa width substantially more than double its depth, said transformerhaving a primary winding and a secondary winding which are spaced apartby the maximum distance permitted by the core, and radiating means inheat conductive contact with the transformer for radiating the heatdissipated within the transformer when on load.

3. A transformer assembly as claimed in claim 2 in it) which theradiating means comprises two metal plates between which the transformeris rigidly situated, the distance between the plates being substantiallyequal to the thickness of the core and one of the windings.

4. A transformer assembly comprising a core and primary and secondarywindings, said core being made of separate laminations of thin foilmaterial, and said Windings being made of thin foil material wound intosubstantially O-shaped form, said windings being placed adjacent oneanother and forming a plate shaped member which passes through and issubstantially completely enclosed by the said core.

5. A transformer assembly as claimed in claim 4 in which the plateshaped member formed by the said substantially O-shaped windings iselongated, having an elongated central opening, the plane of thelaminations of said core being transverse to the length of saidelongated central opening.

6. A transformer assembly as claimed in claim 5 in which each said corelamination is provided with two apertures through which pass parts ofsaid plate shaped member, the parts of said laminations between the twoapertures substantially filling the said elongated central opening.

7. A transformer assembly comprising a core and primary and secondarywindings, said windings being composed of separate laminations of thinfoil material wound into a substantially elongated O-shaped form, onewinding being radially superimposed over the other, and said core beingin two separate portions each made of separate laminations of thin foilmaterial which are wound around one of the elongated sides of theO-shaped windings for complete enclosure.

References Cited in the file of this patent UNITED STATES PATENTS442,649 Talmage Dec. 16, 1890 1,546,885 Burnham July 21, 1925 2,027,405Smede Jan. 14, 1936 2,544,845 Link Mar. 13, 1951 2,635,168 Lerza et a1Apr. 14, 1953 2,702,936 Hurt Mar. 11, 1955 2,764,802 Fiertag Oct. 2,1956 2,771,587 Henperson Nov. 20, 1956 2,840,680 Mills June 24, 19582,879,367 McLean Mar. 24, 1959

1. A TRANSFORMER ASSEMBLY COMPRISING A PLATE-SHAPED ELECTRICAL STEP-DOWNTRANSFORMER HAVING A PRIMARY AND A SECONDARY WINDING, MEANS IN HEATCONDUCTIVE CONTACT WITH THE TRANSFORMER FOR RADIATING HEAT DISSIPATEDWITHIN THE TRANSFORMER AND CONTROL MEANS FOR CAUSING THE TRANSFORMER TOOPERATE UNDER A PREDETERMINED INITIAL LOAD FOR A PREDETERMINED PERIODAND FOR SUBSEQUENTLY DECREASING